Drive Circuit

ABSTRACT

A drive circuit for a high-frequency agitation source includes a signal generator generating a train of low voltage square-wave pulses at a drive frequency, a booster including a boost inductor generating a back EMF and configured to produce a high-voltage pulse train in response to the low-voltage square-wave pulse train and a filter producing a drive signal having a pre-determined harmonic of the drive frequency, the drive signal being used to drive the high-frequency agitation source. The drive circuit is particularly suitable for use with piezoelectric crystals.

The invention relates to a drive circuit for a high-frequency agitationsource. Particularly, the invention relates to a drive circuit for apiezoelectric crystal.

Piezoelectric crystals are well known in the art and are used for anumber of purposes. Piezoelectric motors, transformers and linear drivesare common. An important use for a piezoelectric crystal is innebulisation. There are many cases where a fine mist of a substance isrequired without the application of heat. One example of this is amedical nebuliser, wherein a pharmaceutical compound is nebulised by apiezoelectric crystal in order to be inhaled by a patient. Another usefor nebulisers is in the field of water dispersal such as garden waterfeatures. In order to disperse a dispersal agent effectively, a highvoltage, high frequency drive source is required. Typically, apiezoelectric crystal for use in nebulisation is driven at its resonancefrequency. This frequency varies between piezoelectric crystals, howeverit is usually in the region of 1.6-1.7 MHz.

Drive circuits for piezoelectric crystals are well known in the art. Asimple way of generating such a high frequency signal is through the useof a transistor circuit. However, if this is done, a high voltageamplifier or a transformer is required to generate the peak to peakvoltages needed to drive a piezoelectric crystal. Typically, thesevoltages are in the region of 100-150 V. Transformers are the mostcommonly used components for this purpose. However, they are often bulkyand expensive.

A further requirement for an electronic device that will use a mainspower supply is that Electromagnetic Compatibility Standards (EMC) haveto be met. These standards define an acceptable level for the harmoniccontent in the current which electrical equipment draws from a mains ACsupply, as well as an acceptable level of voltage distortion. Ahigh-voltage square wave signal may contain an unacceptable level ofharmonic content both for efficient driving of a piezoelectric crystaland for meeting the required standards of harmonic content. A common wayof solving this problem is to pass the signal through a low-pass filter.If the low-pass filter is tuned to the fundamental driving frequency ofthe piezoelectric crystal, higher order harmonics can be filtered out,leaving only the fundamental frequency to drive the piezoelectriccrystal. Often, a low-pass filter is also used to give a voltage gain.However, in order to drive a piezoelectric crystal at resonance, arelatively high quality factor is required. In order to achieve thiswith a low-pass filter such as an LC circuit, the capacitances of thesystem in which the LC circuit is located needs to be constant. However,the capacitance of wiring and the piezoelectric crystal itself may varywith temperature, age, condition and use. Therefore, this often makes anLC circuit unsuitable for driving a piezoelectric crystal at the preciseresonant frequency.

The invention provides a drive circuit for a high-frequency agitationsource, the drive circuit comprising signal generating means forgenerating a train of low voltage square-wave pulses at a drivefrequency, boost means including a boost inductor for generating a backEMF, the boost means being arranged to produce a high-voltage pulsetrain in response to the low-voltage square-wave pulse train and filtermeans for producing a drive signal having a pre-determined harmonic ofthe drive frequency, the drive signal being used to drive thehigh-frequency agitation source. Using the back EMF from an inductor togenerate a high-voltage pulse train avoids the use of bulky andexpensive transformers.

Preferably, the high-frequency agitation source is a piezoelectriccrystal.

Advantageously, the filter means comprises a low-pass filter whichincludes an inductor in series with the high-frequency agitation sourceand a capacitor in parallel with the high-frequency agitation source.

The invention provides a simple and cost-effective circuit which is ableto generate a high-voltage, high-frequency, clean sine wave signal todrive a piezoelectric crystal. The invention is particularly suitable todrive a nebuliser for use in a hand dryer.

An embodiment of the invention will now be described with reference tothe accompanying drawings, in which:

FIG. 1 is a circuit diagram of a drive circuit according to theinvention;

FIG. 2 is a graph showing an input signal S2 to a boost stage and ahigh-voltage output signal S3 from the boost stage;

FIG. 3 is a graph showing the cut off frequency of a filter stage;

FIG. 4 a is a graph showing the high-voltage output S3 input to thefilter stage and an output waveform S4 outputted from the filter stage;

FIG. 4 b is an oscilloscope trace showing an actual output waveform S4as supplied to the piezoelectric crystal;

FIG. 5 shows a fast fourier transform of the output waveform S4illustrating the harmonic components of the waveform S4; and

FIG. 6 shows a hand dryer incorporating a nebuliser driven by the drivecircuit of FIG. 1.

FIG. 1 shows a drive circuit according to the invention. The drivecircuit is powered by a DC power source (not shown). The DC power sourceoriginates from an AC/DC converter powered by a mains electricitysupply. The drive circuit comprises three stages: a signal generationstage 1, a boost stage 2 and a filter stage 3. The first stage is thesignal generation stage 1. The signal generation stage 1 comprises amicroprocessor unit MP1 for generating a synchronisation signal at, say,1660 KHz. The microprocessor unit MP1 is supplied at low voltage, forexample 3.3 V. This microprocessor unit MP1 includes a phase-locked loopfor multiplying the synchronisation signal to the required drivefrequency. The output from the microprocessor unit MP1 is connected to apair of complementary push-pull Metal

Oxide Semiconductor Field Effect Transistors (MOSFETs) TR1, TR2. MOSFETTR1 is a low power p-channel MOSFET, and MOSFET TR2 is a low powern-channel MOSFET. The pair of MOSFETs TR1, TR2 provide a push-pulloutput drive. The push-pull arrangement of the MOSFETs TR1, TR2 isrequired to sink and source the gate charge and minimise switchinglosses. The output from the push-pull MOSFETs TR1, TR2 is connected tothe gate of a power MOSFET TR3. The power MOSFET TR3 is supplied by a 5V power rail. The source and drain of the power MOSFET TR3 form part ofthe boost stage 2 and act as a switch in the boost stage 2.

The boost stage 2 comprises an inductor L1, the source/drain of thepower MOSFET TR3 and a capacitor C1. The capacitor C1 is connected inparallel across the source/drain of the power MOSFET TR3. Thesecomponents are connected between the 24 V and ground power rails of thepower source. The inductor L1 has an inductance of 15 μH and thecapacitor C1 has a capacitance of 1 nF.

Connected across the inductor L1 is the filter stage 3. The filter stage3 comprises a low pass filter. The low-pass filter includes an inductorL2 in series with the boost stage 2, and a capacitor C2 in parallel withthe boost stage 2. The capacitance of capacitor C2 and the inductance ofthe inductor L2 are selected such that the resonant frequency of thelow-pass filter is approximately equal to the drive frequency of thepiezoelectric crystal. The capacitor C2 has a capacitance of 2.2 nF andthe inductor L2 has an inductance of 4.7 μH. FIG. 3 shows theattenuation characteristics of the filter stage. These values are chosenin order to provide a 3 dB roll off frequency of approximately 1.6 MHz.Expressed another way, the resonant frequency of the filter stage 3 iscentred on the drive frequency of the piezoelectric crystal according tothe relationship f₀=1/(2π√LC) where L is the inductance of the inductorL2 and C is the capacitance of the capacitor C2. Connected across theoutput from the filter stage 3 is a piezoelectric crystal P1.

In operation, the microprocessor generates a 1660 KHz synchronisationsignal. The phase-locked loop multiplies the synchronisation signal by1024 to generate a drive signal S1 close to 1.7 MHz. The drive signal S1from the microprocessor unit MP1 is then supplied to the complementarypush-pull transistor driver. The MOSFETs TR1, TR2 of the push-pull drivegenerate a square-wave signal S2 which is supplied to the power MOSFETTR3.

The square-wave signal S2 switches the power MOSFET TR3 on or offdepending upon whether the square-wave signal S2 is high or low. Whenthe square-wave signal S2 is high, the power MOSFET TR3 is switched on,the source/drain of the power MOSFET TR3 conducts and completes thecircuit between the 24 V power rail and ground. When this happens, theinductor L1 begins to charge. When the square-wave signal S2 returns toa low state, the power MOSFET TR3 is switched off. This generates alarge rate of change of current in the boost stage 2. The magnetic fieldestablished in the inductor L1 during the on phase of the MOSFET TR3attempts to resist the change in current. This generates a large backEMF in the inductor L1 which produces a high-voltage output signal S3.The high-voltage output signal S3 is shown in FIG. 2. The high-voltageoutput signal S3 consists of a series of peaks which correspond to theback emf generated by the inductor L1. The timing of the leading edgesof the peaks corresponds to the timing of the trailing edges of thesquare-wave signal S2. The high-voltage output signal S3 has the sameduty cycle as the square-wave signal S2. The peak amplitude of thehigh-voltage output signal S3 is in the region of 90 V. The peakamplitude of the high-voltage output signal S3 is limited by thecapacitor C1. The capacitor C1 spreads the energy released by theinductor L1 over a greater time period, reducing the maximum peakvoltage generated. This is required to protect the power MOSFET TR3 fromdamage.

The high-voltage output signal S3 has a high voltage and a pulse periodequal to the inverse of the drive frequency. However, it is not a cleansignal. By this is meant that the high-voltage output signal S3comprises a number of different frequencies in addition to thefundamental frequency. Any waveform or pulse train can be expressed as asuperposition of sine waves of different harmonic frequencies. Thehigh-voltage output signal S3 comprises a large number of unwantedharmonic frequencies. These harmonic frequencies are undesirable becausethey may affect the operation of the piezoelectric crystal and generatea large amount of unwanted harmonic distortion.

In order to remove the unwanted higher harmonic frequencies from thehigh-voltage output signal S3 and leave only the fundamental frequency,the filter stage 3 is used. The filter stage 3 removes the higher orderharmonics present in the high-voltage output signal S3, and the outputS4 from the filter stage 3 is a clean sine wave with a peak-to-peakvoltage of 100-140 V and a drive frequency of 1.7 MHz. FIG. 4 a shows aschematic drawing of the input waveform of the high-voltage output S3and the output waveform S4. FIG. 4 b shows an actual output waveform S4output from the filter stage 3 as “seen” by the piezoelectric crystalP1. The waveform is a sine-wave at the fundamental frequency ofapproximately 1.7 MHz. FIG. 5 shows a fast Fourier transform of thiswaveform. The X-axis shows the frequency (in MHz) and the Y-axis showsthe strength of the harmonic components (in units of dBVrms). The figureillustrates that the low pass filter successfully removes the majorityof the unwanted harmonic frequencies. A component of the second harmonicstill remains, however it is attenuated such that the circuit meets EMCrequirements. The output S4 is then used to drive the piezoelectriccrystal at a frequency of approximately 1.7 MHz.

The above-described embodiment of the invention is a low-cost circuitfor generating a clean, high-voltage, high-frequency sinusoidal waveformfrom a DC source. The invention may be used in any situation where ahigh frequency agitation source is required to be driven cheaply andeffectively. The low component count of the circuit and the absence of atransformer also reduces the physical size of the circuit. This is ofbenefit to applications where size is a crucial factor, for example,household appliances or medical devices.

The above-described embodiment of the invention is particularly suitedfor use in a hand dryer such as that shown in FIG. 6. The hand dryer 100includes a cavity 110. The cavity 110 is open at its upper end 120 andthe dimensions of the opening are sufficient to allow a user's hands(not shown) to be inserted easily into the cavity 110 for drying. Ahigh-speed airflow is generated by a motor unit having a fan (notshown). The high-speed airflow is expelled through two slot-likeopenings 130 disposed at the upper end 120 of the cavity 110 to dry theuser's hands. A drain (not shown) for draining the water removed from auser's hands from the cavity 110 is located at the lower end of thecavity 110. A nebuliser 140 is located downstream of the drain. Thenebuliser 140 is shown partially removed from the hand dryer 100 in FIG.6. The nebuliser 140 is partially cut away to show the location of theabove-described drive circuit 150. The nebuliser 140 includes acollector (not shown) for collecting waste water and a piezoelectriccrystal (not shown) for nebulising the waste water. The piezoelectriccrystal is driven by the drive circuit 150. The low component count andlow cost of the drive circuit means that it is smaller, cheaper tomanufacture and less likely to fail. This means that the size of thehand dryer can be reduced, the reliability of the hand dryer can beimproved and the cost of maintenance is reduced.

It will be appreciated that the invention is not limited to theembodiment illustrated in the drawings. The magnitude and frequency ofthe drive source may be varied depending upon the required application.For example, it is common to drive a piezoelectric crystal at a range offrequencies. However, it is most common to drive a piezoelectric crystalat, or close to, its resonant frequency. For most piezoelectric crystalsthis frequency lies in the range between 1.5 to 2 MHz.

Further, the physical quantities of the described electronic componentsalso may be varied in value. This could be done, for example, to changethe resonant point of the filter stage, or to increase or decrease theback EMF generated by the boost inductor. However, it is desirable thatthe back EMF generated by the boost inductor is greater than 50 V.

There need not be only one low-pass LC filter. The filter stage 3 maycomprise two LC filters in series to attenuate better the higherharmonic frequencies. Further, other forms of signal generator could beused. What is important is that an inductor is used to generate a backEMF to amplify a pulse train, and this signal is then converted into asingle-frequency sine wave using a filter.

1. A drive circuit for a high-frequency agitation source, the drivecircuit comprising signal generating means for generating a train of lowvoltage square-wave pulses at a drive frequency, boost means including aboost inductor for generating a back EMF, the boost means being arrangedto produce a high-voltage pulse train in response to the low-voltagesquare-wave pulse train and filter means for producing a drive signalhaving a pre-determined harmonic of the drive frequency, the drivesignal being used to drive the high-frequency agitation source.
 2. Adrive circuit according to claim 1, wherein the high-frequency agitationsource is a piezoelectric crystal.
 3. A drive circuit according to claim1 or 2, wherein the filter means comprises a low-pass filter.
 4. A drivecircuit according to claim 3, wherein the filter means comprises aninductor in series with the high-frequency agitation source and acapacitor in parallel with the high-frequency agitation source.
 5. Adrive circuit according to any one of the preceding claims, furtherincluding a DC power source and a power rail supplied by the DC powersource.
 6. A drive circuit according to claim 5, wherein the boostinductor is connected in parallel between the power rail of the DC powersource and ground.
 7. A drive circuit according to claim 5 or 6, whereinthe boost means further comprises switching means for switching thepower rail on or off in response to the train of low-voltage square-wavepulses.
 8. A drive circuit according to any one of the preceding claims,wherein the drive frequency is in the region of 1.5-2 MHz.
 9. A drivecircuit according to any one of the preceding claims, wherein the boostinductor generates a back EMF greater than 50 V.
 10. A drive circuit ashereinbefore described with reference to the accompanying drawings. 11.A nebuliser incorporating the drive circuit according to any one of thepreceding claims.
 12. A hand dryer incorporating the nebuliser asclaimed in claim 11.